Shoe press apparatuses used in the press part of a papermaking machine are conventionally classified roughly into two types, one being shown in FIG. 1, and the other being shown in FIG. 2. In both apparatuses, a roll R is disposed in opposition to a shoe SH, and a pair of endless felts F1 and F2, and a shoe press belt 10A, are pinched between the roll and the shoe. A wet paper web P, from which water is to be removed, is held between the endless felts F1 and F2, and passes through a nip press section N provided by the roll R and the shoe SH. Water is removed from the wet paper web P as it passes through the nip. As shown in FIGS. 1 and 2, the roll R and the opposed shoe have conforming shapes, so that they approach each other closely over a relatively wide nip press section N, for a superior water-removing effect.
A relatively long shoe press belt is used in the apparatus of FIG. 1. This shoe press belt travels over a plurality of rolls r (5 rolls in FIG. 1), and is stretched to a predetermined tension. On the other hand, a relatively short shoe press belt is used in the apparatus of FIG. 2.
FIG. 3(a) is a cross-sectional view taken in the cross machine direction through a shoe press belt 10A of the kind conventionally used in a shoe press apparatus of the type shown in FIG. 1 or the type shown in FIG. 2. The belt 10A comprises a base or base body B, a wet paper web side layer 20, which is provided on one side of the base body B (the outer side of the endless loop when in use in a shoe press), and an opposite shoe side layer S, which is on the inner side of the endless loop when in use. The wet paper web side layer 20 and the shoe side layer S are composed of a high molecular weight elastic material. The high molecular weight elastic material is also provided in the base B. The high molecular weight elastic materials forming the shoe press belt 10A are integrated.
The base B is provided to impart strength to the shoe press belt 10A. The base may have any of a variety of constructions. For example, the base may be a woven fabric having a warp and weft, a fabric in which the warp and weft are stacked rather than woven, or a fabric comprising a narrow, strip of non-woven or woven fabric wound in a spiral.
In manufacture of the shoe press belt, the wet paper web side layer 20 and shoe side layer S may be provided on the base body B, either in successive steps or simultaneously. Appropriate high molecular weight elastic materials may be selected from rubber and various other elastomers. Polyurethane resins, and especially thermosetting urethane resins, have been adopted in many cases.
Water-holding concavities 40 are provided in the outer part 11 of the wet paper web side layer 20, for temporarily holding water removed from a wet paper web in a shoe press nip N as shown in FIGS. 1 and 2. The water held in the water-holding concavities 40 is shaken off from a shoe press belt 10A when the direction of travel of the shoe press belt 10A changes.
The water-holding concavities 40 are typically in the form of concave grooves which extend along the machine direction, but may consist of a plurality of separate blind holes formed in layer 20, which are no sufficiently deep to reach the base B. In FIG. 3(a), the water-holding concavities 40 have a cross-sectional shape in which the side walls are straight, and meet the bottoms of the concavities at a right angle. However, the water-holding concavities 40 may have various alternative cross-sectional shapes, as long as they are capable of holding water. For example, the concavities may have curved bottoms as shown in FIG. 3(b), or angled bottoms as shown in FIG. 3(c), or may be in the form of dovetailed grooves, having narrow entrances and larger inside spaces, as shown in FIGS. 3(d), 3(e) and 3(f).
The outer part 11 of the wet paper web side layer comprises not only water-holding concavities 40, but also projecting land sections 50, which are formed in the process of formation of the water-holding concavities 40.
In recent years, papermaking machines have been operated at increased speeds not previously encountered. The nip pressures in the shoe press have also been set to high levels in order to improve the productivity of paper making machines. There has been a need for a shoe press belt which has improved durability so that it is not readily broken under these more severe operating conditions.
When a relatively high pressure is applied to a shoe press belt 10A in the nip of the shoe press during use, a very high compressive load is applied to the belt in the direction of its thickness. Furthermore, a force is applied to the outer part 11 of the wet paper web side layer of the belt, the force being applied to the belt in a direction opposite to the machine direction. The application of a force in a direction opposite to the machine direction results from the fact that, as a part of the belt passes through the nip, a succeeding part is still in the nip. Thus, while the part exiting the nip travels in the machine direction, a load is applied to the succeeding part in the nip in the direction of the belt's thickness. Because this load acts as a braking force on the belt, it generates a load in a direction opposite the machine direction.
In the operation of a paper making machine, the very strong compressive load, which acts in the direction of the belt thickness, and a shear, which acts in the direction opposite to the machine direction, are repeatedly applied to the shoe press belt. These forces cause the high molecular weight elastic material to deteriorate gradually. After a time, the belt will no longer adequately absorb the compressive load and shear, and cracks are generated in the belt.
FIG. 4 is an explanatory view showing where cracks are generated in the case where the wet paper web side layer is composed of a high molecular weight elastic material having a low hardness. Since the hardness of the material is low, the land sections 50 are crushed in the nip, and the shape of the water-holding concavities 40 is warped remarkably. Cracks CR are generated at the corners 43, where the cross-sectional shape of the water-holding concavities 40 changes abruptly. On the other hand, the load applied to the land section 50 in the direction opposite to the machine direction is absorbed to some extent, since the material is flexible.
FIG. 5 is an explanatory view showing where cracks are generated when a wet paper web side layer is composed of a high molecular weight elastic material having a high degree of hardness. In this case, when a load is applied at the nip in the direction of the belt thickness, distortion of the water-holding concavities 40 is unremarkable, since the hardness of the belt is high. Therefore, cracks CR are not frequently generated in the water-holding concavities 40, as they are in the case of FIG. 4. On the other hand, since the hardness of material is high, and the load in the direction opposite to the machine direction may not be adequately absorbed, numerous cracks CR are generated in a surface sublayer 52 of the land section 50.
In view of the above problems, it is an object of the invention to provide a shoe press belt having a high durability, and to prevent the formation of cracks in the surfaces of the land section and at the corners of the water-holding concavities.